This invention relates in general to offset lithographic printing press technology. In particular, the invention relates to an offset lithographic printing press having an improved form roller which eliminates hickeys, ghosting, and other distortions.
A typical offset lithographic printing press has a plate cylinder upon which the negative of the text and illustrations to be printed is etched by a photographic and/or electronic process. Dampening rollers apply water to the plate cylinder, and the water adheres to the plate cylinder except where the text and illustrations are located.
Next, a series of form rollers, also known in the art as inking rollers, apply a layer of ink to the plate cylinder. The ink adheres to the plate cylinder only where the text and illustrations are located.
The plate cylinder then presses the inked text and illustrations onto a rubber blanket cylinder. An impression cylinder then presses a sheet of paper or other material to be printed against the blanket cylinder as the paper or other material passes between the blanket cylinder and the impression cylinder. The inked text and illustrations on the blanket cylinder are transferred onto the paper or other material to effect printing thereof.
One or more of the form rollers may be driven by a vibrating roller. The vibrating rollers may also oscillate laterally to pass the ink more evenly to the form rollers. One or more of the form rollers may be cooled to chill the ink thereby preventing moisture loss.
Impurities and/or clumps of ink on the vibrating rollers may cause the ink to be spread unevenly onto the form rollers. This can cause small dots, commonly referred to as hickeys, to appear on the paper or other material being printed. A related problem involves ghosting of the printed image. Thus, a need exists for a form roller which eliminates hickeys, ghosting, and other distortions that frequently occur in offset lithography.
The present invention comprises a new and improved inking system useful in offset lithography. More particularly, the invention comprises an eccentric, oscillating form or inking roller. The eccentric action increases and decreases pressure between the form roller and the plate cylinder, which in conjunction with oscillating motion (lateral side to side movement of the form roller in a non-uniform manner) eliminates foreign particles and ink clumps thereby substantially reducing hickeys, ghosting, and other distortions of the printed image.
More particularly, the invention includes an eccentric form roller. The form roller is driven through friction circumferentially and laterally by the corresponding vibrating roller. The form roller is marked on the low side of the eccentric. As the form roller is driven circumferentially by the vibrating roller, the contact between the form roller and the plate cylinder changes from minimal pressure to substantial pressure. Simultaneously, one or more cams move or control the movement of the form roller back and forth laterally in a non-uniform manner. The combination of these actions substantially eliminates foreign particles and clumps of ink which eliminates hickeys, ghosting, and other distortions, thereby substantially increasing the quality of the printing.